Abrasive grain has been known for approx. 75 years and has been manufactured on a commercial scale for approx. 30 years. Thus, U.S. Pat. Nos. 1,871,792 and 1,871,793 describe the back-filling of a pouring stream of liquid corundum by means of compressed air or steam, creating hollow corundum spheres approx. 0-5 mm in diameter. In the above-referenced US documents all essential steps currently applied in the manufacturing process have already been described. Numerous additional patents describe the use of hollow spheres created by this process. DE 628 936, for example, describes the pulverization and glazing of the hollow spheres into abrasive granules that are subsequently manufactured into grinding tools.
Owing to the high porosity of the material and the low firmness of the hollow spheres, the processing into abrasive grain yields relatively fine grain, suitable only for a limited number of uses. However, with fine-graining necessarily occurring during the conventional processing of fused corundum, the manufacture of abrasive grains from hollow-sphere corundum makes little economic sense.
The manufacturing process of conventional abrasive grain corundum begins with alum or bauxite being melted in an electric arc furnace. Subsequently, the melted corundum is cooled, having produced blocks that weigh 10 to 15 tons. Following a 2- to 5-day complete cooling process, these corundum blocks are crushed by crushers and rollers and the resulting material is then filtered out as abrasive grains. Therefore, an essential portion of the costs involved in producing corundum is attributed to the crushing process and wear and tear sustained while crushing corundum blocks having diameters up to several meters and then abrasive grains ranging in size from a few micrometers to a few millimeters. Therefore, attempts were made in the past time and again to avoid the expensive crushing procedure by dispersing the liquid melted corundum as early as prior to its solidification.
As mentioned above, however, back-filling a pouring stream of melted metal consisting of liquid corundum creates porous hollow-sphere corundum that is ill suited as abrasive grain.
Thus, because of its low thermal conductivity, chemical inertia, and porosity, it is currently used mainly as fire resistant material. Utilization of spherical corundum by the refractory industry was first described in U.S. Pat. No. 2,261,639.
By contrast, the abrasives industry uses spherical corundum mainly as pore-forming material in the production of grinding wheels, rather than abrasive grain. Such grinding tools are, for example, described in U.S. Pat. No. 2,986,455 or DE 1 281 306.
Regardless, back-filling of corundum was resumed time and again as a means to manufacture abrasive grain. U.S. Pat. No. 2,261,639, for example, describes melted material consisting of aluminum oxide with an additive of 1-10% sodium oxide, back-filled with air, yielding compact crystalline spheres that can be utilized as abrasive material after being crushed. Large portions of sodium oxide produce sodium aluminates, substantially reducing the performance of the abrasive grain.
U.S. Pat. No. 2,340,194 describes the addition of 1-1.5% of titanium oxide in the melted material, supposed to produce compression-proof hollow spheres with relatively thick walls. DD 134 638 describes a method for the production of corundum hollow spheres, whereby the characteristics of the corundum hollow sphere are affected by added nitrite-bound nitrogen in 5 the form of aluminum nitride or aluminum oxynitride.
GB 284 131 describes a method in which liquid corundum is first blown into a cooling chamber by means of airflow, where individual particles undergo additional cooling by a flow of cool water. This produces particles of approx. 3 mm in diameter. EP 1 157 077 describes the production of polycrystalline abrasive grains, where the liquid corundum is cast and the cooling process is aided by the dispersion of the melted aluminum oxides into small drops with the use of ultrasound. This method yields dense particles with a medium diameter of less than 1 mm. The crystallite property of these particles ranges below 30 μm.
The methods of the state of the art mentioned have the disadvantage that they either attempt to influence the physical characteristics such as porosity and density of the desired product by means of chemical composition, logically resulting in a change in the chemical composition of the product, thereby often reducing the performance of the product as an abrasive grain, or the attempt is made to affect the physical properties of the product by way of technical procedural variations, which generally involves a great increase in technical effort, resulting in high production costs, so that to date none of the above-mentioned methods of a direct production of abrasive grain has been successful.